1. Field of the Invention
This application lies in the field of sonar instrumentation. More particularly, it is concerned with sonar apparatus which is utilized for shallow depth prospecting for fish. More particularly, it is concerned with the use of sonar apparatus in conjunction with a display-recorder, which may be of linear or curvilinear form.
2. Description of the Prior Art
The concept of noise suppression has been patented previously by Austin U.S. Pat. No. 3,747,053. There has been attempts to provide signal amplitude differentiation. Both concepts have been used separately in sonars for some time, but no one has been able to make them work together. The uniqueness of this invention is that it combines these two concepts, which are normally incompatible, into a new circuit which accomplishes both variable noise suppression and signal amplitude differentiation at the same time.
Variable noise suppression is a method of pulse width discrimination. The sonar tone burst transmission is normally of very short duration, approximately 200 usec. Unfortunately, there are other sources of sound energy in the same frequency range from sources other than the sonar. These signals when received by the sonar are called noise. Noise typically occurs as pulses of short duration. Therefore, if the pulse width of the transmitted tone burst is increased and an integrator is added to the receiver which requires the reception of a tone burst for a given length of time before it is accepted as a legitimate signal, the noise pulses of short duration will be lost. A potentiometer on a sonar called a suppression control determines the pulse width of the transmitted pulse and the integration time of the receiver. It is typically set only high enough to remove noise signals since the higher it is set, the lower the resolution of the sonar.
Signal amplitude differentiation has also been used. When a received signal in a sonar chart recorder goes above a predetermined amplitude, a signal differentiator causes the recording mechanism to inhibit printing or display of the signal. When the received signal falls below that threshold, printing or display of the signal resumes. The amplitude differentiator is delayed a short length of time so that the leading edge of the signal is always seen. What this results in is that the bottom of the body of water being charted appears as a thin black line after which is a white area. The bottom signal resumes printing after its level falls below a predetermined level. This function is useful for determining where the bottom is underneath structure which does not reflect as strong a signal as the bottom.
These two concepts have been incompatible with each other in the past. This is because the suppression circuit creates a delayed version of the original signal which causes the leading edge of the bottom signal, the "thin black line" to be lost when in the signal differentiation mode. Time delays are critical to both circuits and they work against each other.